The invention discloses a high-concentration low-viscosity water-soluble macromolecular sizing and a production method thereof, wherein the method comprises the steps of: at first, activating raw cassava starch with an activating agent, then performing catalytic degradation on the starch by using combined acid, simultaneously adding composite adjuvant, and performing high-temperature treatment for 1 to 9 hours in a frying tank through oligomerization, catalytic reaction and other processes to obtain the product. The entire procedure adopts a dry method chemical modification process for development and production, and is environmentally-friendly and free from pollution. The product has the characteristics of stable performance, high concentration, low viscosity, good fluidity, strong cohesive force and high drying speed. As an adhesive for paper products, the high-concentration low-viscosity water-soluble macromolecular sizing can meet the demands of high concentration and low viscosity. The sizing can be used independently or together with other adhesives, is suitable for the enterprises with conditions for the production of modified starch, and is produced by using the frying tank and a powder picking machine without particular requirement on production equipment.